1. Field of the Invention
The present invention relates to an apochromatic lens system which can be corrected for chromatic aberration at a plurality of wavelengths. In particular, the present invention relates to an apochromatic lens system which can be used as a projection lens system. The system is especially suitable for forming a photomask pattern on a substrate of a semiconductor device (such as an integrated circuit).
2. Description of the Prior Art
Exposing through a patterned photomask is one process for fabricating a semiconductor device. In the exposing step, a photoresist coating a semiconductor substrate is exposed to light which is transilluminated through a photomask having a predetermined mask pattern. Generally, a plurality of photomasks, each having an individual mask pattern, are necessary to fabricate a single semiconductor device. Further, the exposing step must be performed for each photomask. Thus, the photomask must be aligned so that its pattern is matched with a pattern previously formed on the semiconductor substrate. In the prior art, a microscope is used to observe the alignment of the photomask.
As a rule, photoresist material is highly sensitive to light with a short wavelength but not sensitive to light with a long wavelength. Therefore, light with a long wavelength (e.g., 546.07 nm (e line)) is used to align the photomask, whereas light with a short wavelength (e.g., 435.83 nm (g line)) is used to expose the photoresist.
Thus, in general, when photomask patterns are formed on a semiconductor substrate, the photomasks are aligned with long wavelength light. Thereafter, the exposing step is performed with short wavelength light. Hence, it is desirable to use a lens system which is corrected for chromatic aberration at the long wavelength as well as at the short wavelength. If a lens system has chromatic aberration at the long wavelength and at the short wavelength, an image of the mask pattern during alignment differs from that during exposure, along an optical axis. In such a case, for each alignment and exposure, either the photomask or the semiconductor substrate must be moved along the optical axis to be in focus. The photomask or the substrate often slips while being moved along the optical axis. As a consequence, all work devoted to alignment is in vain.
On the other hand, to reduce exposure time and improve efficiency, it is desirable to use light with a plurality of wavelengths. For example, exposing a photoresist using light with a wavelength of 435.83 nm (g line) and light with a wavelength of 404.66 nm (h line) is more favorable than exposing using only the g line. However, when lights with two different short wavelengths, e.g., the g line and the h line, are used, a lens system must be corrected for chromatic aberration at at least the g line and the h line.
Therefore, it is advantageous if a photomask and a substrate do not have to be moved along the optical axis for each alignment and exposure. To achieve this, a lens system must be corrected for chromatic aberration at a long wavelength (long wavelength light is used during alignment) and two different short wavelengths (short wavelength lights are used during exposure). However, it is extremely difficult to obtain such a lens system.